1. Field of the Invention
This invention relates to a synchronous motor with field coil excitation equipped with a power-electronic interface for a regenerating braking system and a regenerating braking system including the same.
2. Description of the Related Art
As a synchronous motor with field coil excitation, an alternator of a lundell motor for regenerating a part of vehicle engine power and generating electricity is known and is used with almost all automobile vehicles. Recently, the alternator has been also used as a motor generator operating as a starter at the engine starting time. The synchronous motor with field coil excitation is widely used for a railway vehicle motor, an elevator motor, a servo motor, a large-scaled generator of a power plant, etc.
The efficiency of a motor reaches near 90% and steady efficiency improvement approaches the limit. In contrast, large regenerative power occurs at the motor decelerating time and if the power is not regenerated, the energy loss is 30% or more; it reaches 50% or more for a servo motor which is frequently started and stopped. Therefore, there is a demand for use of the power in power regeneration and power running using an electricity storage device.
Power regeneration technology at the decelerating time using a synchronous motor advances in an automobile including a battery as an electricity storage device. In an alternator of an automobile, a hybrid automobile, etc., regenerative power is recovered into a battery through an inverter from stator coil at the decelerating time.
JP-A-2005-143157 describes a motor controller for converting an alternating current (ac) into a direct current (dc) through an inverter from a stator coil, regenerating power in an electricity storage device, and supplying a field current to a rotor coil after adjusting a voltage with a DC/DC converter from the electricity storage device (see page 1 and FIG. 1).
JP-A-2005-94922 describes a power supply system for transmitting a part of electric power stored in a high-voltage battery from an engine and a motor from a rotor coil to a stator coil in a low-voltage generator with field coil excitation and charging a low-voltage battery to provide low-voltage electric power for driving auxiliaries, etc., of a vehicle (see page 1 and FIG. 1).
In the related-art motor controller described in JP-A-2005-143157, however, the regenerative braking efficiency of the field current supply of the rotor coil results from multiplying the efficiency of the inverter by the charge and discharge efficiency of the electricity storage device by the efficiency of the DC/DC converter, which inevitably results in low regenerative braking efficiency.
In the power supply system described in JP-A-2005-94922, when the rotor rotates or when the rotor stops, the rotor coil and the stator coil are assumed to be a transformer and the electric power is only transmitted from the rotor coil to the stator coil and it is impossible to even enhance the regenerative braking efficiency of the motor.
Further, in the related-art apparatus described above, there are a problem in that if an instantaneous regenerative braking current is allowed to flow into a battery, it cannot be stored as electric power because the current acceptance ratio of the battery is low and most of the instantaneous regenerative power is converted into heat, and a problem in that the life of the electricity storage device is shortened as the instantaneous regenerative braking current is allowed to flow into the battery. For example, for a lead storage battery, the current acceptance ratio is low and an instantaneous regenerative braking current becomes heat, overheating the lead storage battery and shortening the life of the battery. Additionally, to use a lithium ion battery, there is a problem in that if a regenerative braking current is accepted in a high charge state, the positive electrode becomes a high potential and remarkable degradation occurs. To prevent degradation, a method of bypassing only the instantaneous current and discarding it as heat with a resistor is available; however, the method leads to a high cost and also results in further lowering of the regenerative braking efficiency.
Thus, in the related-art regenerating braking system including the synchronous motor with field coil excitation, an alternating current is converted into a direct current through the inverter from the stator coil, power is regenerated in the electricity storage device, and a field current is supplied to the rotor coil after a voltage is adjusted with the DC/DC converter from the electricity storage device. Thus, the efficiency of the field current supply of the rotor coil results from multiplying the efficiency of the inverter by the charge and discharge efficiency of the electricity storage device by the efficiency of the DC/DC converter, which inevitably results in low motor efficiency and low regenerative braking efficiency. There is also a problem in that the life of the electricity storage device is shortened as the instantaneous regenerative braking current is allowed to flow into the electricity storage device.